Our invention is adapted to be used in a multiple ratio planetary transmission situated in a vehicle driveline having an internal combustion engine with a throttle control and a hydrokinetic torque converter situated between the engine and input elements of the gearing.
The gearing comprises two simple planetary gear units arranged in a manner similar to the gearing arrangement of the well-known Simpson gear set. Located between the turbine of the torque converter and the input elements of the Simpson gearing is a simple planetary gear unit with a friction clutch adapted to connect two elements of the third gear unit together for rotation in unison. A friction brake also is used for anchoring a reaction element of the third planetary gear unit. An overrunning coupling establishes one-way torque flow between two elements of the gearing. The brake is arranged in series relationship with respect to the clutch.
A second overrunning coupling in a gear unit of the Simpson gearing is used for the purpose of establishing non-synchronous ratio shifts. Forward engagement is achieved by engaging a forward clutch on a shift from neutral to a drive state. Similarly, a separate reverse engagement clutch is used to establish a torque flow path for reverse. In each instance, turbine speed is used as a feedback signal to initiate the start of the forward or reverse engagement.
Ratio changes between the first ratio and the second ratio on an upshift, as well as on a downshift from the second ratio to the first ratio, are achieved in our improved transmission by controlling the engagement and release of an overrunning coupling. The overrunning clutch is arranged in series relationship with respect to a friction brake as a reaction torque flow path for the friction brake associated with the intermediate ratio is established and disestablished. The braking of the friction brake is accomplished with a closed loop control so that harshness is avoided as the overrunning elements of the reaction torque flow path engage. This is in contrast to prior art arrangements, such as that shown in U.S. Pat. No. 5,157,608, where a non-synchronous shift using overrunning couplings is achieved without the cushioning effect made available by the present invention as the associated friction brake is actuated.
Our improved control system controls a 2-1 downshift, for example, by controlling the disengaging element, which is an intermediate ratio brake band, and engaging the overrunning coupling for low ratio. The overrunning coupling engagement is controlled by partially holding the clutch torque over the downshift interval, and it does this in a closed loop fashion.
Ratio changes between higher ratios are controlled in our improved transmission by controlling the application and release of brake servos in two operating steps rather than a single step. The ratio shift that requires the application of the intermediate servo on a 3-2 downshift or a 4-2 downshift, for example, is achieved using two intermediate steps following the command of a shift until the completion of the shift. This is done by having a set of flow control orifices in communication with the intermediate servo during a downshift in the first step that is different than the orifice selection introduced during the downshift in the second step. Ratio changes between overdrive ratio and direct drive ratio and downshifts from overdrive ratio to intermediate ratio involve the control of the overrunning coupling for overdrive. This control strategy distinguishes the present invention from the control strategy for the transmission described in U.S. Pat. No. 5,383,825, issued Jan. 24, 1995, to Joseph E. El-Khoury, Frank W. Timte, Edmond R. League, and Gerard P. Kuchta. That patent is assigned to the assignee of the present invention.
Features of the control strategy shown in U.S. Pat. No. 5,383,825 have been applied in our improved transmission to control ratio changes between third and fourth ratio and between first and second ratio wherein one input parameter for the ratio change controller is the percentage of shift completion. The control system includes a forward clutch pressure modulator valve that acts in cooperation with a 1-2 upshift valve to establish an accumulator effect that cushions the application of the intermediate speed ratio servo. Intermediate servo apply pressure is established as the clutch modulator valve feeds pressure through a control orifice to the apply side of the intermediate servo as the exhaust side of the servo is exhausted through a reverse engagement control valve and reverse engagement modulator. In the case of a 2-1 downshift, the shift solenoid is deactivated causing the 1-2 shift valve to move to the downshift position. Intermediate servo apply pressure then is released through the 1-2 upshift valve and the 2-3 upshift valve to an exhaust port. This causes the intermediate servo to lose capacity.
The pressure made available to the intermediate servo by the forward clutch modulator valve can vary from zero to the maximum stall pressure. Thus, low capacity as well as high capacity 1-2 shifts can be executed. The point at which the inertia phase of the shift begins is dependent on the magnitude of the forward clutch modulator pressure, which is readily calibrated. Thus, a limited amount of slip during a shift can be achieved with a minimum transient torque disturbance during the shift. This is equivalent to an accumulator system of the conventional type in which an accumulator piston would be installed on the release side of the intermediate servo. A fast release of oil from the intermediate servo is prevented. This accumulator feature is entirely electronically controlled by means of software.
On a 2-3 upshift, the control system relies upon a 3-2 downshift KD control valve that connects the intermediate servo release pressure chamber with the high clutch and prevents a pre-filling of the intermediate servo release chamber. The high clutch and the intermediate servo are exhausted through the 3-2 downshift TD control valve (and/or the MAN1 timing valve) during a 3-2 downshift. The transmission assumes its intermediate stage for a calibratable amount of time thus establishing an electronic accumulator effect. The pressure on the intermediate servo release side can be varied from zero to a corresponding maximum high clutch modulated capacity.
The separate downshift steps on a 3-2 downshift reduces the complexity of the control assembly by applying pressure from the clutch pressure modulator valve to the intermediate servo apply side. Any capacity requirement can be achieved on the high clutch. Only two shift valves are required to accomplish the two different stages as two different orifice sizes for a 3-2 downshift are used, the two shift valves controlling the pressure flow pattern through the orifices of various sizes.
On a 4-3 shift, the electronic control system controls the overdrive band capacity upon engagement of an overrunning coupling which transmits torque during third ratio. On a 4-3 downshift, in order to improve shift feel, the torque flow components ahead of the overrunning coupling are accelerated until the overrunning clutch engages. The overdrive band capacity during the downshift at the engagement point for the overrunning coupling will be reduced. When the overdrive band is completely released, the overrunning coupling will carry full input torque.
On a 3-4 shift, modulated pressure from the clutch pressure modulator valve is applied to the overdrive servo and the pressure will vary from zero to a maximum stall pressure. When the inertia phase begins during a 3-4 upshift, the output shaft torque characteristic is totally dependent on the magnitude of the clutch pressure modulator pressure characteristic. A minimum torque disturbance is achieved with a limited amount of slip during the shift by controlling the clutch pressure modulator valve output pressure. This accumulator function is achieved without the requirement for adding hardware elements, such as an accumulator piston and accumulator spring as in conventional control systems.
The accumulator effect is achieved entirely electronically by means of software on both a 2-1 downshift and a 4-3 downshift. The percentage shift completion value during a shift is monitored, and the duration of an electronic pressure control ramp is determined electronically by the microprocessor using the value for the percentage of shift completion as an input. The overdrive servo is allowed to hold partial torque during the inertia phase of a 4-3 downshift, thereby improving the clutch engagement quality for the associated overrunning coupling.
The multiplexing of the functions of the pressure modulator valve for forward drive, the shift solenoid valves, the orifices and timing valves reduce the complexity of the valve system needed to achieve control of ratio changes between overdrive ratio and intermediate speed ratio and between low speed ratio and intermediate speed ratio. These different control functions are achieved with common valve components.